It is well known that incorporation of a nucleating agent (NA) in semicrystalline thermoplastic polymers such as polyolefins (PO) increases crystallization temperature, reduces spherulitic size and improves clarity and mechanical properties of the polymer. Generally, nucleating agents are high melting compounds which do not melt at processing temperature of the polymer and remain as discrete particles embedded in polymer melt. These insoluble particles behave as nucleator for the crystallization of polymer melt and generate innumerable crystallization sites resulting innumerous small spherulites some nucleating agents are soluble in polymer melt but they solidify before crystallizatin of polymer melt and, thus, generate sites for crystallization. Addition of nucleating agents in polyolefins reduces the size of spherulites and improves clarity. It also enhances mechanical properties, increases crystallization temperature and reduces cycle time for injection moulding process.
A large variety of both organic and inorganic compounds have been used as nucleating agents. Most widely used nucleating agents are benzylidene derivatives of sorbitols, metal salts of organic carboxylic and sulfonic acids and phosphate salts. Of these nucleating agents, sorbitol derivatives are highly effective in improving transparency. However, use of sorbitol derivatives is not preferred in extrusion because, it tends to sublime during extrusion causing contamination of equipment parts and it gives odor. Moreover, they are very expensive. Hence the metal salts or organic acids, though less effective, are preferred in many application. Metallic salts of carboxylic acid belong to non-soluble category of nucleating agents. Sodium benzoate is the example of nucleating of nucleating agent belonging to metal salt of carboxylic acid class.
Metal salts of different mono, di and poly carboxylic aliphatic, substituted and un substituted aromatic acids, carboxylic acids containing cyclic and fused rings with alkali/alkaline earth metal belonging to IA, IIA and IIIA group in periodic table have been used. U.S. Pat. Nos. 5,912,292, and 3,852,237 describe the use of sodium benzoate as nucleating agent. U.S. Pat. Nos. 5,714,531 and 5,998,576 describe the use of rosin acid salts as nucleating agents. Rosin is naturally occurring material containing mixtures of acids such as abietic acid, dehydroabietic acid and many other acids. Metallic salts of dehydorabietic acid are reported to be effective nucleating agents. Shellac is another naturally occurring compound containing many carboxylic acids. However, it is not used as nucleating agent.
Shellac is resinous material obtained from insects. It is composed of mainly hydroxy fatty acids like aleuritic acid, butolic acid and other mono and id hydroxy acids along with hydroxy terpenic acids like jalaric acid, shelloic acid and laksholic acids. Aleuritic acid and jalaric acid and/shelloic acid are the major constituents of shellac. Jalaric acid is easily converted to shelloic acid. Aleuritic acid having formula (1)
is a monobasic acid containing three hydroxyl groups whereas shelloic acid of formula (2)
is a hydroxy terpenic acid having fused ring system. Salts of carboxylic acid containing fused ring system have low volatility.